As shown in Japanese Unexamined Patent Application Publication No. 2014-136211, for example, there is known a honeycomb structure having partition walls as catalyst carriers to purify exhaust gases discharged from automobiles or the like. The partition wall has a polygonal lattice form. The honeycomb structure has a plurality of cells forming flow channels of the exhaust gases which are formed to be surrounded by the partition walls. The honeycomb structure is mounted in an exhaust pipe. When the high-temperature exhaust gases flow in the plurality of cells, a temperature of the partition wall rises, and catalyst carried on the partition wall is activated. Thereby, harmful substances in the exhaust gases are purified.
The plurality of cells are divided into inner and outer cells. The inner cells are formed on an inner part of the honeycomb structure, which is disposed around a central axis of the outer skin. The outer cells are formed outside of a region where the inner cells are formed. The individual opening areas of the inner cells are smaller than those of the outer cells. Thereby, a flow resistance of the exhaust gases in the inner cells is set to be larger than that of the outer cells to equalize a velocity of the respective exhaust gases in the inner and the outer cells. A separator is disposed between respective regions having the inner and the outer cells which are sectioned thereby. The separator is made up of the partition wall.
When the honeycomb structure is mounted in the exhaust pipe, the honeycomb structure, surrounded by a mat, is pressed into the exhaust pipe. Therefore, a large external force in a radial direction of the outer skin is applied to the honeycomb structure from the mat. So-called incomplete cells, which are surrounded by the partition walls and the separator, are formed in a vicinity of the separator. Therefore, when the honeycomb structure is pressed into the exhaust pipe, stress tends to concentrate on the separator and in its vicinity. That is to say, the separator and in its vicinity are the weakest against stress in the honeycomb structure. Therefore, in the honeycomb structure, while a thickness of the separator is increased so as to enhance a strength of the separator, the partition wall in the vicinity of the separator is thickly formed. Thereby, the strength of the entire honeycomb structure is enhanced, and the resistance to stress prevents the partition wall from being damaged.
However, in the honeycomb structure, when the thickness of the partition wall in the vicinity of the separator is increased, a weight of the honeycomb structure tends to increase. Therefore, it is possible to enhance the strength of the entire honeycomb structure, but the catalyst carried on the partition wall is heated and activated less quickly by the exhaust gases when increasing the weight of the honeycomb structure. Therefore, making a lighter-weight honeycomb structure is expected to activate the catalyst quickly.
Further, in the honeycomb structure, when the thickness of the partition wall in the vicinity of the separator is increased, opening areas of cells in the vicinity of the separator tend to be small. Therefore, the flow resistance of the exhaust gases becomes high, and pressure loss of the exhaust gases tends to increase.
In addition, after the honeycomb structure is produced, a process to form a catalyst layer on the partition wall is carried out by applying a catalyst slurry including noble metals to a surface of the partition wall. In the honeycomb structure, when the thickness of the partition wall in the vicinity of the separator is increased, the opening area of the cell in the vicinity of the separator tends to be small. Therefore, the cell is likely to be clogged by the catalyst. Therefore, the pressure loss of the exhaust gases tends to increase even more. Further, since the exhaust gases do not flow in the clogged cell, the catalyst in the clogged cell does not contribute to purifying the exhaust gases.